1. Field of the Invention
The present invention relates to a tire pressure drop detecting method and device for detecting whether the tire pressure drops or not,based on the rotational angular velocities of four tires provided for a four-wheel vehicle.
2. Description of Related Art
As one of the safety devices for a four-wheel vehicle such as automobile, truck, etc., devices for detecting a tire pressure drop (DWS) have recently been developed, and some of them have been put to practical use.
As one of methods of detecting a tire pressure drop, for example, there is a method of utilizing a difference in the respective rotational angular velocities F.sub.1, F.sub.2, F.sub.3 and F.sub.4 of four tires W.sub.1, W.sub.2, W.sub.3 and W.sub.4 provided for a vehicle. This method utilizes the fact that the rotational angular velocities F.sub.i (i=1, 2, 3, 4) vary depending on the state of the pneumatic pressure of the tire W.sub.i. That is, when the pneumatic pressure of any tire W.sub.i drops, the effective rolling radius of the tire W.sub.i decreases. As a result, the rotational angular velocity F.sub.i of the tire W.sub.i becomes larger. Therefore, the pressure drop of the tire W.sub.i can be detected based on the difference in rotational angular velocities F.sub.i of four tires W.sub.i.
Incidentally, the effective radius is a value obtained by dividing by 2 .pi. a travel distance of the vehicle while the tires make one free revolution.
One embodiment of a method of determining the pressure drop of a tire W.sub.i based on the difference in rotational angular velocities F.sub.i is as shown in the following equation (1) (e.g. see Japanese Laid-Open Patent Publication (KOKAI) Nos. 63-305011 and 4-212609). ##EQU1##
If the effective rolling radiuses of four tires W.sub.i are equal, the respective rotational angular velocities F.sub.1 are equal (F.sub.1 =F.sub.2 =F.sub.3 =F.sub.4). Accordingly, the judged value D becomes 0. Then, threshold values D.sub.TH1 and D.sub.TH2 are set (provided D.sub.TH1, D.sub.TH2 &gt;0). If the condition shown in the following expression (2) is satisfied, it is judged that the tire W.sub.i whose pneumatic pressure has dropped is present. If this condition is not satisfied, it is judged that all of the tires W.sub.i have a normal inner pressure. EQU D&lt;-D.sub.TH1, or D&gt;D.sub.TH2 (2)
However, there is a fear of causing an erroneous judgment according to the traveling state of the vehicle only by performing the pneumatic pressure drop judgment using the above expressions (1) and (2). For example, in case that the pneumatic pressure of any one of the right front and left front tires W.sub.i, the tires to which a driving force is transmitted, has dropped, there is a fear of causing an erroneous judgment when the vehicle is traveling at comparatively high speed without decelerating (hereinafter referred such a traveling as to a "high-speed drive traveling").
That is, at the time of high-speed drive traveling, the slip rate of the tire W.sub.i whose pneumatic pressure has dropped is lowered, while the effective rolling radius is increased by a centrifugal force applied to the respective parts of the tire W.sub.i. Therefore, the difference in rotational angular velocity between a tire W.sub.i having a normal inner pressure and a tire W.sub.i whose pneumatic pressure has dropped nearly disappears. As a result, the judged value D approaches zero in the form of the second order function as the speed V of the vehicle increases even if the pneumatic pressure of any tire W.sub.i has dropped, and hardly satisfy the condition of the above expression (2).